I have a 2003 CTS and I live in Ohio, so its cold here. When I let my car warm up in the mornings, its gets nice and warm in there (I normally let it run for about 15-20 minutes) but when i get about 2-3 miles down the road it starts to blow cold to warm air. When i then get to my destination and let it sit for a few minutes it begins to get warm again. I just had a Thermostat put on and had the radiator and heater core flushed like a month ago. So why would this car not blow hot air when i drive? I do keep both driver and passenger control settings on 90 degrees when i am driving and turn the A/C to off when i first start the car. Any help would be a benifit to me. Mind you i am not a mechanic so please use understanding terms with me, since i may not know what you might be refering to. Thanks in advanced to whom can help me with this issue.. Also, would it be wise to put cardboard behind the grill to cut back on the cold air coming in on the motor? Someone at work told me to do that and wasn't sure if that would help the situation..........